1. Field of the Invention
The present invention relates generally to a magnetic bubble memory sense amplifier and, more particularly, to a phase tolerant magnetic bubble sense amplifier.
2. Prior Art Statement
In a typical form, magnetic bubble memory systems include a magnetic bubble memory device in which magnetic bubbles are stored and retrieved under the control of an appropriate control circuit. During the bubble retrieval operation, the control circuit moves the magnetic bubbles stored in the magnetic bubble memory device in a conventional manner so that a previously-stored seqence of bubbles and voids are passed beneath a detector resistor formed on the surface of the device. In response to the movement of a magnetic bubble therebeneath, the detector resistor briefly changes in resistance value, as compared to a dummy resistor which is also formed on the magnetic bubble memory device to have substantially the same nominal resistance as the detector resistor. This induced differential voltage change is detected by a sense amplifier constructed to provide an output signal when the differential exceeds a predetermined threshold voltage. Typically, however, the sense amplifier must identify the desired differential voltage change in the presence of a substantial common-mode signal produced by the access field and other high-frequency noise, as well as an almost unavoidable offset voltage.
In the past, attempts were made to utilize sense amplifiers which were designed for other applications, such as the Motorola MC1444 Plated Wire Sense Amplifier, to perform the magnetic bubble detection function. Dissatisfaction led to the development of special sense amplifiers for use in magnetic bubble memory systems, such as the Texas Instruments SN75281 Bubble-Memory Sense Amplifier. However, such sense amplifiers have been less than fully satisfactory due to excessive offset voltage sensitivity, undesirable input impedance loading characteristics, or other similar deficiencies. More recently developed sense amplifiers, such as those used in the Intel 7242 Formatter/Sense Amplifier, promise some improvement, but have yet to be fully evaluated.
Relevant available materials describing the prior art circuits referenced above are submitted herewith. In addition, an overview of magnetic bubble memory systems and their operations may be found in the following U.S. Pat. Nos. 4,152,776; 4,156,935; and 4,159,537.